If you have enough disk space copy the NMIS4 RRD database to the NMIS8 folder, if not use the configuration to have NMIS8 point to the NMIS4 RRD database

run a collect on a single node, and you should see NMIS8 go into auto conversion mode which will convert existing NMIS4 configuration files to NMIS8, and update some RRD files.

Ideally you should perform a test upgrade on a separate server, but if you have a backup you can roll back your upgrade if you need to.

There are a few other considerations around platforms, e.g. 64bit Linux is recommended for NMIS8, it is likely that you are currently running an older version of 32bit Linux. It is possible to install a brand new NMIS8 server and copy the NMIS4 data and convert it ready for NMIS8, there are a few extra steps than the above.

Opmantek has support services which can assist you to migrate should you need assistance.